Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Calculations for basic and diluted earnings per common share
Net income	$ 57,983	$ 49,091	$ 114,253	$ 91,625
Preferred stock dividends	(2,339)	(2,169)	(4,669)	(4,367)
Net income available to common equity	55,644	46,922	109,584	87,258
Common shareholder dividends	(18,326)	(16,443)	(36,577)	(32,646)
Unvested share-based payment awards	(110)	(182)	(227)	(388)
Undistributed earnings	37,208	30,297	72,780	54,224
Undistributed earnings allocated to common shareholders	36,980	29,963	72,273	53,649
Undistributed earnings allocated to unvested share-based payment awards	228	334	507	575
Undistributed earnings	37,208	30,297	72,780	54,224
Basic
Distributed earnings to common shareholders	18,326	16,443	36,577	32,646
Undistributed earnings allocated to common shareholders	36,980	29,963	72,273	53,649
Total common shareholders earnings, basic	55,306	46,406	108,850	86,295
Diluted
Distributed earnings to common shareholders	18,326	16,443	36,577	32,646
Undistributed earnings allocated to common shareholders	36,980	29,963	72,273	53,649
Total common shareholders earnings, diluted	$ 55,306	$ 46,406	$ 108,850	$ 86,295
Weighted average common shares outstanding (in shares)	151,573	148,511	151,196	148,556
Effect of dilutive common stock awards (in shares)	1,958	1,019	2,155	962
Effect of dilutive common stock warrants (in shares)	771	0	796	0
Diluted weighted average common shares outstanding (in shares)	154,302	149,530	154,147	149,518
Basic earnings (loss) per common share (in usd per share)	$ 0.36	$ 0.31	$ 0.72	$ 0.58
Diluted earnings (loss) per common share (in usd per share)	$ 0.36	$ 0.31	$ 0.71	$ 0.58